A Caltech-led team of astronomers has discovered evidence for this scenario taking place within a fiercely energetic object known as a quasar. Quasars are active cores of galaxies in which a supermassive black hole is siphoning material from a disk encircling it. In some quasars, the supermassive black hole creates a jet that shoots out at near the speed of light. The quasar observed in the new study, PKS 2131-021, belongs to a subclass of quasars called blazars in which the jet is pointing toward the Earth. Astronomers already knew quasars could possess two orbiting supermassive black holes, but finding direct evidence for this has proved difficult.

Reporting in The Astrophysical Journal Letters, the researchers argue that PKS 2131-021 is now the second known candidate for a pair of supermassive black holes caught in the act of merging. The first candidate pair, within a quasar called OJ 287, orbit each other at greater distances, circling every nine years versus the two years it takes for the PKS 2131-021 pair to complete an orbit.

The telltale evidence came from radio observations of PKS 2131-021 that span 45 years. According to the study, a powerful jet emanating from one of the two black holes within PKS 2131-021 is shifting back and forth due to the pair’s orbital motion. This causes periodic changes in the quasar’s radio-light brightness. Five different observatories registered these oscillations, including Caltech’s Owens Valley Radio Observatory (OVRO), the University of Michigan Radio Astronomy Observatory (UMRAO), MIT’s Haystack Observatory, the National Radio Astronomy Observatory (NRAO), Metsähovi Radio Observatory in Finland, and NASA’s Wide-field Infrared Survey Explorer (WISE) space satellite.

The combination of the radio data yields a nearly perfect sinusoidal light curve unlike anything observed from quasars before.

“When we realized that the peaks and troughs of the light curve detected from recent times matched the peaks and troughs observed between 1975 and 1983, we knew something very special was going on,” says Sandra O’Neill, lead author of the new study and an undergraduate student at Caltech who is mentored by Tony Readhead, Robinson Professor of Astronomy, Emeritus.

Ripples in Space and Time

Most, if not all, galaxies possess monstrous black holes at their cores, including our own Milky Way galaxy. When galaxies merge, their black holes “sink” to the middle of the newly formed galaxy and eventually join together to form an even more massive black hole. As the black holes spiral toward each other, they increasingly disturb the fabric of space and time, sending out gravitational waves, which were first predicted by Albert Einstein more than 100 years ago.

The National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory), which is managed jointly by Caltech and MIT, detects gravitational waves from pairs of black holes up to dozens of times the mass of our sun. However, the supermassive black holes at the centers of galaxies have millions to billions of times as much mass as our sun, and give off lower frequencies of gravitational waves than those detected by LIGO.

In the future, pulsar timing arrays — which consist of an array of pulsing dead stars precisely monitored by radio telescopes — should be able to detect the gravitational waves from supermassive black holes of this heft. (The upcoming Laser Interferometer Space Antenna, or LISA, mission would detect merging black holes whose masses are 1,000 to 10 million times greater than the mass of our sun.) So far, no gravitational waves have been registered from any of these heavier sources, but PKS 2131-021 provides the most promising target yet.

In the meantime, light waves are the best option to detect coalescing supermassive black holes.

The first such candidate, OJ 287, also exhibits periodic radio-light variations. These fluctuations are more irregular, and not sinusoidal, but they suggest the black holes orbit each other every nine years. The black holes within the new quasar, PKS 2131-021, orbit each other every two years and are 2,000 astronomical units apart, about 50 times the distance between our sun and Pluto, or 10 to 100 times closer than the pair in OJ 287. (An astronomical unit is the distance between Earth and the sun.)

Revealing the 45-Year Light Curve

Readhead says the discoveries unfolded like a “good detective novel,” beginning in 2008 when he and colleagues began using the 40-meter telescope at OVRO to study how black holes convert material they “feed” on into relativistic jets, or jets traveling at speeds up to 99.98 percent that of light. They had been monitoring the brightness of more than 1,000 blazars for this purpose when, in 2020, they noticed a unique case.

“PKS 2131 was varying not just periodically, but sinusoidally,” Readhead says. “That means that there is a pattern we can trace continuously over time.” The question, he says, then became how long has this sine wave pattern been going on?

The research team then went through archival radio data to look for past peaks in the light curves that matched predictions based on the more recent OVRO observations. First, data from NRAO’s Very Long Baseline Array and UMRAO revealed a peak from 2005 that matched predictions. The UMRAO data further showed there was no sinusoidal signal at all for 20 years before that time — until as far back as 1981 when another predicted peak was observed.

“The story would have stopped there, as we didn’t realize there were data on this object before 1980,” Readhead says. “But then Sandra picked up this project in June of 2021. If it weren’t for her, this beautiful finding would be sitting on the shelf.”

O’Neill began working with Readhead and the study’s second author Sebastian Kiehlmann, a postdoc at the University of Crete and former staff scientist at Caltech, as part of Caltech’s Summer Undergraduate Research Fellowship (SURF) program. O’Neill began college as a chemistry major but picked up the astronomy project because she wanted to stay active during the pandemic. “I came to realize I was much more excited about this than anything else I had worked on,” she says.

With the project back on the table, Readhead searched through the literature and found that the Haystack Observatory had made radio observations of PKS 2131-021 between 1975 and 1983. These data revealed another peak matching their predictions, this time occurring in 1976.

“This work shows the value of doing accurate monitoring of these sources over many years for performing discovery science,” says co-author Roger Blandford, Moore Distinguished Scholar in Theoretical Astrophysics at Caltech who is currently on sabbatical from Stanford University.

Like Clockwork

Readhead compares the system of the jet moving back and forth to a ticking clock, where each cycle, or period, of the sine wave corresponds to the two-year orbit of the black holes (though the observed cycle is actually five years due to light being stretched by the expansion of the universe). This ticking was first seen in 1976 and it continued for eight years before disappearing for 20 years, likely due to changes in the fueling of the black hole. The ticking has now been back for 17 years.

“The clock kept ticking,” he says, “The stability of the period over this 20-year gap strongly suggests that this blazar harbors not one supermassive black hole, but two supermassive black holes orbiting each other.”

The physics underlying the sinusoidal variations were at first a mystery, but Blandford came up with a simple and elegant model to explain the sinusoidal shape of the variations.

“We knew this beautiful sine wave had to be telling us something important about the system,” Readhead says. “Roger’s model shows us that it is simply the orbital motion that does this. Before Roger worked it out, nobody had figured out that a binary with a relativistic jet would have a light curve that looked like this.”

Says Kiehlmann: “Our study provides a blueprint for how to search for such blazar binaries in the future.”

Video: https://youtu.be/B_q9tYjvgiY

The Astrophysical Journal Letters study titled “The Unanticipated Phenomenology of the Blazar PKS 2131-021: A Unique Super-Massive Black hole Binary Candidate” was funded by Caltech, the Max Planck Institute for Radio Astronomy, NASA, National Science Foundation (NSF), the Academy of Finland, the European Research Council, ANID-FONDECYT (Agencia Nacional de Investigación y Desarrollo-Fondo Nacional de Desarrollo Científico y Tecnológico in Chile), the Natural Science and Engineering Council of Canada, the Foundation for Research and Technology — Hellas in Greece, the Hellenic Foundation for Research and Innovation in Greece, and the University of Michigan. Other Caltech authors include Tim Pearson, Vikram Ravi, Kieran Cleary, Matthew Graham, and Tom Prince. Other authors from the Jet Propulsion Laboratory, which is managed by Caltech for NASA, include Michele Vallisneri and Joseph Lazio.

Plastic waste residing in either landfill or the natural environment currently outweighs all living biomass (4 Giga tonnes), culminating in one of the great environmental challenges of the 21^st century. Whilst recycling rates are increasing across Europe, traditional methods remain limited because the harsh remelting conditions reduce the quality of the material each time it’s recycled.

Now researchers at the CSCT have developed a mild and rapid chemical recycling process for polycarbonates, a robust class of plastics commonly used in construction and engineering.

Using a zinc-based catalyst and methanol, they were able to completely break down commercial poly(bisphenol A carbonate) (BPA-PC) beads within 20 minutes at room temperature.

The waste can then be converted into its chemical constituents, namely bisphenol A (BPA) and dimethyl carbonate (DMC), helping to preserve product quality over an infinite number of cycles.

Importantly, BPA recovery prevents leakage of a potentially damaging environmental pollutant, whilst DMC is a valuable green solvent and building block for other industrial chemicals.

Their results are published in ChemSusChem, noting enhanced process efficiency and milder conditions compared to previous methods.

Promisingly, the catalyst is also tolerant to other commercial sources of BPA-PC (e.g. CD) and mixed waste feeds, increasing industrial relevance, whilst being amenable to other plastics (e.g. poly(lactic acid) (PLA) and poly(ethylene terephthalate) (PET)) at higher temperatures.

The team has also demonstrated a completely circular approach to producing several renewable poly(ester-amide)s (PEAs) based on terephthalamide monomers derived from waste PET bottles. These materials have excellent thermal properties and could potentially be used in biomedical applications, for example drug delivery and tissue engineering.

Lead researcher Professor Matthew Jones, at the University of Bath’s CSCT, said: “It’s really exciting to see the versatility of our catalysts in producing a wide range of value-added products from plastic waste.

“It’s crucial we target such products, where possible, to help promote and accelerate the implementation of emerging sustainable technologies through economic incentives.”

First author of the paper, Jack Payne from the CSCT, said: “Whilst plastics will play a key role in achieving a low-carbon future, current practices are unsustainable.

“Moving forward, it’s imperative we source plastics from renewable feedstocks, embed biodegradability/recyclability at the design phase and diversify existing waste management strategies.

“Such future innovation should not be limited to emerging materials but encompass established products too.

“Our method creates new opportunities for polycarbonate recycling under mild conditions, helping to promote a circular economy approach and keep carbon in the loop indefinitely.”

Presently, the technology has only been demonstrated on a small scale, however, the team is now working on catalyst optimisation and scaling up the process (300 mL) with collaborators at the University of Bath. The research was funded by the Engineering and Physical Sciences Research Council.

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A study from the University of California, Davis, and NOAA Fisheries’ Southwest Fisheries Science Center confirms a striking decrease in basking shark sightings in the CCE after the 1970s and 1980s and examines what is driving their presence and distribution. The work is published in the journal Frontiers in Marine Science.

The findings hold implications for the global conservation of basking sharks, which are classified as endangered by the International Union for Conservation of Nature’s Red List.

Red flags

Very little is known about basking sharks, including how long they live, where they mate or where they give birth. Population assessments have not been conducted in the CCE, which stretches from Baja to British Columbia, and there is no formal monitoring effort around the species. But what data exists serves as a red flag that the species warrants further investigation.

“They are a rare sight,” said lead author Alexandra McInturf, a Ph.D. candidate with the UC Davis Department of Wildlife, Fish and Conservation Biology at the time of the study and currently a postdoctoral fellow at Oregon State University. “We want to know why the declines are happening. Is it climate change? Human-induced pressures? What environmental cues do they respond to and how might that change in the future?”

For this study, the authors examined the longest dataset available — systematic aerial surveys for small fish conducted by NOAA Fisheries between 1962 and 1997, and additional sources between 1973 and 2018 that include fisheries data, tagging and research efforts and public observations.

Aerial surveys sighted a maximum of 4,000 basking sharks in 1965. After the 1990s, no sightings were reported. Additional sources reported less than 100 sightings per year after 1990. School sizes also declined by about half between the 1960s and 1980s, from 57 to 24 individuals per group. In the decades following, no schools larger than 10 were reported.

“While the aerial surveys were focused on small fish, the survey also collected data on basking sharks, which turned out to be invaluable,” said co-author Heidi Dewar, who oversees the project for Southwest Fisheries Science Center. “Currently, fishing for basking sharks is prohibited in the U.S., Mexico and Canada. The hope is that with these protections in place, the population will recover. Additional work to understand the sources of mortality in international waters is also needed.”

Seasonal shifts

The study also found a significant shift in when sightings occurred — from fall and spring in earlier years of the surveys to the summer months after the 2000s.

What’s driving these trends is still unclear, although researchers found a significant link between shark presence and environmental factors, such as sea surface temperature, prey presence, El Niño and other climatic oscillations.

The study notes a lack of monitoring may also play a role in reduced sightings and that more research is needed to characterize their full range.

Suggestions for conservation

The study concludes with suggestions for conservation of the species, noting that coastal areas from Monterey Bay to Baja California remain important habitat for basking sharks. The suggestions include:

• Coordinating the documentation of basking shark deaths and sightings throughout the Pacific basin to create more robust population estimates.
• Increased monitoring of shark fin markets.
• Developing region-specific genetic markers to help ensure that regulations for international trade in endangered species are being followed.

McInturf said increased public awareness of the sharks may also help. For instance, they are susceptible to ship strikes as they feed on the surface, so boater education is an important part of their conservation.

How to identify a basking shark

Basking sharks, with their massive mouths and bodies, were once thought to be sea monsters and have been mistaken for great white sharks due to their similar body structure.

“We get questions like, ‘Can they swallow a person?'” said McInturf. “No, their throats are too small.”

The basking shark has much smaller prey in mind. Like whales, it filter-feeds on tiny phytoplankton to fill its sizable belly.

To identify a basking shark, a big dorsal fin is a key giveaway, and a second smaller dorsal fin may also be visible. As they feed, their tail or nose may surface. And unlike most sharks’ more direct swimming pattern, basking sharks tend to zig-zag or snake through the water.

After three seasons of field work in Ireland to study basking sharks, McInturf has seen them only once, breaching out of the water.

“That tells you what it’s like to work with basking sharks,” McInturf said. “I would love to see them in the water again. I’ve spent so much time looking for them, and they’ve been incredibly hard to find.”

The study’s additional co-authors include Barbara Muhling and Joseph Bizzarro of UC Santa Cruz and NOAA’s Southwest Fisheries Science Center, David Ebert of the Pacific Shark Research Center (Moss Landing Marine Labs), and Nann Fangue and Damien Caillaud of UC Davis.

The research was supported by California Sea Grant, the UC Davis Agricultural Experiment Station and NOAA’s Southwest Fisheries Science Center.

“To our knowledge, this is the first longitudinal study of oral health and e-cigarette use. We are now beginning to understand how e-cigarettes and the chemicals they contain are changing the oral microbiome and disrupting the balance of bacteria,” said Deepak Saxena, who led the research with Xin Li; both are professors of molecular pathobiology at NYU College of Dentistry.

Gum disease affects nearly half of U.S. adults over 30 years of age. Smoking cigarettes is a known risk factor for developing gum disease, but less is known about the impact of e-cigarettes — which vaporize nicotine and other chemicals — on oral health, especially the long-term consequences of vaping.

The researchers studied the oral health of 84 adults from three groups: cigarette smokers, e-cigarette users, and people who have never smoked. Gum disease was assessed through two dental exams six months apart, during which plaque samples were taken to analyze the bacteria present.

Changes to gum health

All participants had some gum disease at the start of the study, with cigarette smokers having the most severe disease, followed by e-cigarette users. After six months, the researchers observed that gum disease had worsened in some participants in each group, including several e-cigarette users.

A key indicator of gum disease is clinical attachment loss, measured by gum ligament and tissue separating from a tooth’s surface, leading the gum to recede and form pockets. These pockets are breeding grounds for bacteria and can lead to more severe gum disease. In a study of the same participants published in Frontiers in Oral Health, the research team found that clinical attachment loss was significantly worse only in the e-cigarette smokers — not non-smokers and cigarette smokers — after six months.

A unique microbiome

The researchers then analyzed the bacteria found in the plaque samples and determined that e-cigarette users have a different oral microbiome from smokers and nonsmokers — building on findings the team previously reported in iScience and Molecular Oral Microbiology.

While all groups shared roughly a fifth of the types of bacteria, the bacterial makeup for e-cigarette users had strikingly more in common with cigarette smokers than nonsmokers. Several types of bacteria, including Selenomonas, Leptotrichia, and Saccharibacteria, were abundant in both smokers and vapers compared to nonsmokers. Several other bacteria — including Fusobacterium and Bacteroidales, which are known to be associated with gum disease — were particularly dominant in the mouths of e-cigarette users.

When plaque samples were gathered and analyzed in the six-month follow-up, the researchers found greater diversity in bacteria for all groups studied, yet each group maintained its own distinct microbiome.

“Vaping appears to be driving unique patterns in bacteria and influencing the growth of some bacteria in a manner akin to cigarette smoking, but with its own profile and risks to oral health,” said Fangxi Xu, a junior research scientist in Saxena’s lab and the study’s co-first author.

An altered immune response

The researchers found that the distinct microbiome in e-cigarette users was correlated with clinical measures of gum disease and changes to the host immune environment. In particular, vaping was associated with different levels of cytokines — proteins that help regulate the immune system. Certain cytokines are linked to an imbalance in oral bacteria and can worsen gum disease by making people prone to inflammation and infection.

TNFα, a cytokine that causes inflammation, was significantly elevated among e-cigarette users. In contrast, cytokines IL-4 and IL-1β were lower among e-cigarette users; IL-4 tends to be reduced in people with gum disease and increases after treatment, which suggests that certain bacteria in the mouths of e-cigarette users may be actively suppressing immune responses.

The researchers concluded that the distinct oral microbiome of e-cigarette users elicits altered immune responses, which along with clinical markers for gum disease illustrate how vaping presents its own challenge to oral health.

“E-cigarette use is a relatively new human habit,” said Scott Thomas, an assistant research scientist in Saxena’s lab and the study’s co-first author. “Unlike smoking, which has been studied extensively for decades, we know little about the health consequences of e-cigarette use and are just starting to understand how the unique microbiome promoted by vaping impacts oral health and disease.”

This research was supported by the National Institute of Dental & Craniofacial Research (DE025992, DE027074), the National Cancer Institute (CA206105), and the NYU Mega-Grants Initiative. The study included additional collaborators from NYU College of Dentistry, NYU Grossman School of Medicine, and the University of Pennsylvania.

In the study, published in Environmental Research Letters, researchers found a high probability of flood damage — including monetary damage, human injury and loss of life — for more than a million square miles of land across the United States across a 14-year period. That was more than 790,000 square miles greater than flood risk zones identified by FEMA’s maps.

“We’re seeing that there’s a lot of flood damage being reported outside of the 100-year floodplain,” said the study’s lead author Elyssa Collins, a doctoral candidate in the NC State Center for Geospatial Analytics. “There are a lot of places that are susceptible to flooding, and because they’re outside the floodplain, that means they do not have to abide by insurance, building code and land-use requirements that could help protect people and property.”

It can cost FEMA as much as $11.8 billion to create national Flood Insurance Rate Maps, which show whether an area has at least a 1% chance of flooding in a year, according to a 2020 report from the Association of State Floodplain Managers. Researchers say their method of using machine learning tools to estimate flood risk offers a way of rapidly updating flood maps as conditions change or more information becomes available.

“This is the first spatially complete map of flood damage probability for the United States; wall-to-wall information that can be used to learn more about flood risk in vulnerable, underrepresented communities,” said Ross Meentemeyer, Goodnight Distinguished Professor of Geospatial Analytics at NC State.

To create their computer models, researchers used reported data of flood damage for the United States, along with other information such as whether land is close to a river or stream, type of land cover, soil type and precipitation. The computer was able to “learn” from actual reports of damage to predict areas of high flood damage likelihood for each pixel of mapped land. They created separate models for each watershed in the United States.

“Our models are not based in physics or the mechanics of how water flows; we’re using machine learning methods to create predictions,” Collins said. “We developed models that relate predictors — variables related to flood damage such as extreme precipitation, topography, the relation of your home to a river — to a data set of flood damage reports from the National Oceanic and Atmospheric Administration. It’s very fast — our models for the U.S. watersheds ran on an average of five hours.”

The actual flood damage reports they used to “train” the models were publicly available reports from NOAA made between December 2006 and May of 2020. Compared with recent FEMA maps downloaded in 2020, 84.5% of the damage reports they evaluated were not within the agency’s high-risk flood areas. The majority, at 68.3%, were located outside of the high-risk floodplain, while 16.2% were in locations unmapped by FEMA.

When they ran their computer models to determine flood damage risk, they found a high probability of flood damage for more than 1.01 million square miles across the United States, while the mapped area in FEMA’s 100-year flood plain is about 221,000 square miles. Researchers said there are factors that could help explain why the differences were so large, including that their machine-learning-based model assessed damage from floods of any frequency, while FEMA only includes flooding that would occur from storms that have a 1% chance of happening in any given year.

“Potentially, FEMA is underestimating flood damage exposure,” Collins said.

One of the biggest drivers of flood damage risk was proximity to a stream, along with elevation and the average amount of extreme precipitation per year. The three Census regions with the highest probability were in the Southeast. Louisiana, Missouri, the District of Columbia, Florida and Mississippi had the highest risk of any U.S. state or district in the continental United States. Of the 30 most high-risk counties, North Carolina had three: Dare, Hyde and Tyrrell.

In their model, researchers used historical climate data. In the future, they plan to account for climate change.

In the meantime, researchers say their findings, which will be publicly accessible, could be useful for helping policymakers involved in land-use planning. They also represent a proof-of-concept method for efficiently updating flood maps in the future.

“There is still work to be done to make this model more dynamic,” Collins said. “But it’s part of a shift in thinking about how we approach these problems in a more cost-effective and computationally efficient manner. Inevitably, with climate change, we’re going to have to update these maps and models as events occur. It would be helpful to have future estimates that we can use to prepare for whatever is to come.”

The study, “Predicting flood damage probability across the conterminous United States,” was published online Feb. 21, 2022, in Environmental Research Letters. In addition to Collins and Meentemeyer, other authors included Georgina M. Sanchez, Adam Terando, Charles C. Stillwell, Helena Mitasova and Antonia Sebastian. This project was supported by the U.S. Geological Survey Southeast Climate Adaptation Science Center (G19AC00083) and the North Carolina State University Sea Grant program (R/MG-2011).